1. Field of the Invention:
The present invention relates to the preparation of dihydroxybenzenes and, more especially, to the preparation of dihydroxybenzenes by the hydroxylation of phenols.
2. Description of the Prior Art:
The hydroxylation of phenol or of substituted phenols using hydrogen peroxide to prepare diphenols has long been known to this art.
French Patent FR No. 2,071,464 describes a process in which such reaction is catalyzed by a strong acid, such as, for example, perchloric acid or sulfuric acid.
German Patent No. 2,410,742 describes a process similar to that of the '464 patent, in which the hydrogen peroxide is employed in the form of a virtually anhydrous organic solution.
The above two processes have attracted considerable interest and the former is used industrially.
However, for a considerable period of time, this art has sought to catalyze the hydroxylation reaction by means of solids which are undissolved in the reaction medium, in order to simplify their separation therefrom, to permit the optional recycling thereof, and to avoid saline byproducts which are principally formed during removal of the dissolved acid catalysts.
Thus, French Patent FR No. 2,489,816 recommends the use of titanium silicalite as a heterogeneous catalyst for the hydroxylation of aromatic compounds by means of hydrogen peroxide.
However, such catalysis presents extreme problems of reproducibility. Moreover, the fine size of these catalyst particles makes their separation from the reaction medium very difficult and their recycling problematical, while in an industrial process, it is essential to be able to recycle an expensive catalyst.
In order to remedy this problem of separation of the catalyst, it has been proposed, in the European Patent Application published under No. 200,260, to employ agglomerates of these fine particles of titanium silicalite.
The difficulty in controlling the reliability of plant installations when using hydrogen peroxide for hydroxylation and the mediocrity of the prior art yields have long prompted those skilled in this art to seek, essentially unsuccessfully, to introduce a hydroxyl group directly onto the phenolic ring in the absence of any peroxide derivative.
The only publication describing the direct introduction of a hydroxyl group onto a benzene nucleus is believed to be the article by Iwamoto, published in the Journal of Physical Chemistry, 87, 6 (1983).
Such benzene hydroxylation reaction is carried out by means of nitrous oxide (N.sub.2 O) in the presence of a catalyst based on an oxide of a metal of Groups V or VI of the Periodic Table.
Vanadium oxide is the preferred oxide among the oxides of the metals of Groups V and VI of the Periodic Table. It is more preferred to employ such oxide distributed on a substrate based on silica, in an amount by weight ranging from 1% to 10% relative to the substrate. The substrate preferably is constituted of silica, since alumina in most cases causes the formation of a mixture of the oxides of carbon.
This particular process was of interest, but the requirement for special catalysts made it relatively unattractive to industry.
Thus, serious need continues to exist in this art for a process for the direct hydroxylation of the benzene nucleus on a simple and readily available substrate.